1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus which has a polishing unit for polishing a workpiece such as a semiconductor wafer, a washing unit for washing the workpiece which has been polished and a universal transfer robot for transferring the workpiece.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Although the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, such a polishing apparatus has a single function of polishing a semiconductor wafer. Therefore, in the case of washing a semiconductor wafer after polishing, the semiconductor wafer is required to be transferred or transported from the polishing apparatus to a washing apparatus. Further, in the case of polishing a semiconductor wafer again under a different condition after a first polishing operation, the semiconductor wafer is required to be transferred or transported from one polishing apparatus to another polishing apparatus. In these cases, the semiconductor wafers are manually transferred or transported by a movable container in which they are immersed in water to keep them from drying during transportation. However, since various apparatuses including a polishing apparatus and a washing apparatus are independently installed and the semiconductor wafers are transferred or transported by the movable container containing water therein, it is difficult to install the polishing apparatus, the washing apparatus and the like in a clean room of a semiconductor manufacturing plant and to automate various processes including a polishing process and a washing process completely.
In order to solve the above problems, there has been proposed an apparatus which has a polishing unit and a washing unit housed in a common housing. Further, if necessary, there is a possibility that a plurality of polishing units are housed in a common housing.
In the polishing apparatus which has a polishing unit and a washing unit, or a plurality of polishing units in a common housing, it is conceivable to construct a cluster type of polishing apparatus which integrates a plurality of units including a polishing unit and a washing unit, as adopted in a semiconductor manufacturing process such as etching or chemical vapor deposition (CVD).
However, in the case of constructing a cluster type of polishing apparatus which integrates a plurality of units and incorporates a universal transfer robot at a central position of the units, it is necessary to handle a dirty and wet semiconductor wafer soiled with abrasive slurry or particles generated by the polishing operation or a clean and dry semiconductor wafer which is placed on a loading unit or an unloading unit. Therefore, a conventional robot incorporated in a cluster type of a semiconductor manufacturing processing apparatus cannot be used in a cluster type of polishing apparatus because the robot cannot handle a clean semiconductor wafer and a dirty semiconductor wafer separately. If the conventional robot is incorporated in the polishing apparatus, a washing process and a drying process of the robot or a robot arm are additionally required, thus lowering throughput.
Further, in the case of leaving the robot or the robot arm for a long time as it is, abrasive material or particles generated by the polishing operation adheres to the robot or the robot arm, resulting in contaminating of subsequent semiconductor wafers or respective units of the polishing apparatus.